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Page:Popular Science Monthly Volume 4.djvu/331

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Leyden jar furnished a number of single sparks, each time the coil was excited, the number varying between one and thirty, according to circumstances. The whole proceeding consumed an interval of time often as great as one-fiftieth of a second; that is, the jar loaded up and discharged itself twenty or thirty times in that period. Prof. Rood found the number of elements of the spark to vary with its length, the nature of the electrodes, and the size of the jar. Short sparks are more complex than long ones, small jars give more than large ones, and metallic points a greater number than balls. The point to be determined was, the duration of the several elements of the spark, and especially of its quickest element. In one case of a discharge lasting the fiftieth of a second, it began with an ordinary spark, followed by a pale-violet light, lasting about one-sixtieth of a second, and then came a compact

Fig. 5.
PSM V04 D331 Spark images graphic.jpg
Images of Spark drawn out.

body of ten or twenty sparks, this last act continuing for about one two-hundredth of a second. The results of the inquiry are thus stated by Prof. Rood: "From the foregoing, then, it appears that, if a jar, having a metallic coating of about one hundred square inches, be connected, as above described, with an induction-coil, its discharge will be effected by a considerable number of acts, of which the first is by far the most intense. Further, the metallic particles, heated up by the first discharge to a white heat, almost instantly assume a lower temperature, marked by a corresponding change from white to brownish yellow; and, as their temperature continues to fall, the tint changes, in the case of brass electrodes, to green; in that of platinum, to a gray or violet-gray. These observation's further demonstrated the fact that four ten-millionths of a second is an interval of time quite sufficient for the production of distinct vision."

It was also shown that the first act of the electric explosion, represented by the white band, lasted through an interval of time so short as to be immeasurable. It was proved that it could not occupy more